1. Field of the Invention
The present invention refers to a driving circuit for a voice coil motor and driving method thereof.
2. Background of the Invention
In the state of the art electronic circuits utilized to move reading and writing heads in disk memory system of the type used in computer systems and similar are known. In such circuits a coil is widely used as an actuator to move and position the reading and writing heads; such a coil actuator is typically called a xe2x80x9cvoice coil motorxe2x80x9d or even simpler xe2x80x9cvoice coilxe2x80x9d due to its similarity to the coils commonly used in audio speakers and the like. The voice coils are operated in a way similar to the acoustic coils of the audio speakers. In fact, a positive current applied to the coil produces a movement of the reading and writing head in one direction, while a negative current produces a movement of the head in the opposite direction.
The voice coils are operated in two different operational modes. The first mode is the so-called seek mode (seek) wherein large currents are applied to the coil to rapidly bring the reading and writing head to the point or track of the disk where the desired information which must be read or written is located.
The second mode is the track-follow mode (track-follow) wherein small currents are applied to the coil to maintain the position of the reading and writing head in the desired track or point.
The systems used have to be linear so that the acceleration of the reading and writing head is directly proportional to the value of the current applied to the coil. Therefore, because of the high degree of linearity required and because of the large currents required for the seek mode, the amplifiers employed in the voice coil systems operate as class AB amplifiers.
A circuit scheme of a known voice coil system is shown in FIG. 1. A voice coil motor indicated by a coil L1, which belongs to a driving mechanism of a reading and writing head of a computer hard disk, is supplied by a current delivered by two class AB power amplifiers A10 and A20, the first one of which is in non-inverting configuration and the second one is in inverting configuration. The current 11 which passes through the coil L1 is directly proportional to the input voltage Vin coming from a digital-analog converter D. The magnetic field generated by the coil L1 allows the reading and writing head to move. A resistor Rs is placed in series to the coil L1 to the terminals of which the inputs of a sense operational amplifier O1 the output voltage signal of which is added with the signal Vin at the inverting input of a error differential operational amplifier ErI are connected. The non inverting input of the operational amplifier ErI is connected to a reference voltage Vref while the output is connected to the input of the power amplifiers A10 and A20; the outputs of the amplifiers A10 and A20 are driven in inverted phase by the output voltage signal Outl of the amplifier ErI. The presence of the amplifier O1 and the error amplifier ErI assure a certain precision in the work state of the reading and writing head.
In view of the state of the art described, it is an object of the present invention to provide a driving circuit for a voice coil motor which assures a higher precision in the control of the current passing through it and also a higher precision in the placement of the reading and writing head above all in track-follow mode.
According to present invention, such object is obtained by means of a control circuit providing a driving current to a voice coil motor to position a reading and writing head of a disk memory system, said circuit comprising a first and a second class AB amplifier, the outputs of which are connected to the terminals of a first resistor in series with said voice coil motor so that a current passes through said voice coil motor and through said first resistor, said circuit comprising a sense amplifier the input terminals of which are coupled with the terminals of said first resistor, a device at the input of which a signal is present which is a sum of an external signal and of an output signal of the sense amplifier, said first amplifier and said second amplifier being driven in inverted phase by an output signal produced by said device, characterized by comprising means for inserting a second resistor in series with the first resistor so that said current passes through the series of said first and said second resistors and so that said sense amplifier has the input terminals coupled with the terminals of said series of resistors.
According to present invention it is possible to provide a driving method of a voice coil motor.
The features and the advantages of the present invention will be made evident by the following detailed description of an embodiment thereof, shown as not limiting example in the annexed drawings.